Type Functions: Implementation Status

Open bugs related to type families

Declarations involving families:

#2677 (depending on the order of instances, overlap is sometimes not properly recognised)

TestSimple8:

Fix tcLookupFamInst to gracefully handle this case. (This requires some care to not violate assumptions made by other clients of this function, as it is also used for data families, but I see no fundamental problem.)

Issue a warning if there are two identical instances (as per Roman's suggestion).

Defaults for associated type synonyms. (Having both a kind signature and vanilla synonym is problematic as in RnNames.getLocalDeclBinders its hard to see that not both of them are defining declarations, which leads to a multiple declarations error. Defaults are quite different from vanilla synonyms anyway, as they usually have tyvars on their rhs that do not occur on the lhs.) If an associated synonym has a default definition, use that in the instances. In contrast to methods, this cannot be overridden by a specialised definition. (Confluence requires that any specialised version is extensionally the same as the default.)

Can we add a warning in case of a signature that is ambiguous due to a type variable only been mentioned as an argument to a type synonym family?

To fix superclass equalities (specifically getting the coercion evidence), we could introduce a kind of typelet just for evidence. In fact, re-use HsBind.VarBind and make its right-hand side a specially data structure describing evidence construction, instead of being a general HsExpr. That evidence construction generation can have a case for extracting superclass constraints. The desugarer than has to generate the case expression bringing the equality in scope from that.

What about filtering the EqInsts in TcSimplify.addSCs. We need them, don't we? But they give rise to Vars, not Ids, and we haven't got selectors.

Rank-n types: In TcTyFuns.flattenType, we need to pull out type families below foralls.

It'd be nice if the error message of tcfail167 would include "Inaccessible case alternative: Can't match types Char' and Float'" again. We could achieve in TcPat.tcConPat by having a look at eq_preds in the GADT case. If the eq_preds are obviously unsatisfiable (due to a clash of data type constructors), then we have an inaccessible case alternative.

Rigidity:

Shiuld test GADT7 really succeed? or is it ok to fail.

Remove the dodgy rigidity test that is in tcConPat right now.

implement the proposal where we infer a rigidity flag for case scutinees and pass that down when type checking the patterns.

We infer the rigidity flag for the case scrutinee by generalising its type and checking whether that has any foralls at the top. It's rigid if it has no foralls.

As usual, if a pattern has a GADT constructor (ie, any constraints in the data constructor signature), the scutinee must be rigid,

We need to know of types whether they are rigid (not only whether they contain unification variables). We can achieve that by a flag in the environment that indicates whether the computation of that type involved non-rigid type variables.

When a type instance changes (in an orphan modules), currently clients are not properly recompiled at least by --make.

When we raise a mismatch error in TcSimplify for unresolvable equalities, we effectively tidy the two non-matching types twice. Add a comment to highlight this and say why it is ok (i.e., they are never grouped together with groupErrs or similar).

#2296: error message involving fundep gives unhelpful location. I want to remember to come back to this one when we have the new type-family simplification stuff in place.

Step 1: Replace the existing improvement machinery for FDs by code that generates explicit equalities from the two FD rules. Then, all improvement is by normalisation of equalities, which hopefully allows us to simplify TcSimplify.reduceContext. (Apply this change when integrating the simplification of equalities and dictionaries.)

It seems a bit complicated to come up with the most general type. The relevant code is in TcExpr.tcExpr in STEP 4 of the RecordUpd case.

Parsing and Renaming

Todo (low-level): None.

Todo (high-level):

Defaults for associated type synonyms. (Having both a kind signature and vanilla synonym is problematic as in RnNames.getLocalDeclBinders its hard to see that not both of them are defining declarations, which leads to a multiple declarations error. Defaults are quite different from vanilla synonyms anyway, as they usually have tyvars on their rhs that do not occur on the lhs.)

Done:

Parsing and renaming of kind signatures (toplevel and in classes).

Parsing and renaming of indexed type declarations (toplevel and in classes).

Using new syntax with family and instance on top level.

Added -findexed-types switch.

Allowing type tag in export lists to list associated types in the sub-binder list of an import/export item for a class.

Import/export lists: ATs can be listed as subnames of classes and the data constructors of instances of a data family are subnames of that family.

Parsing and renaming of equational constraints in contexts.

Type Checking

If an associated synonym has a default definition, use that in the instances. In contrast to methods, this cannot be overridden by a specialised definition. (Confluence requires that any specialised version is extensionally the same as the default.)

Todo (high-level):

Type checking in the presence of associated synonym defaults. (Default AT synonyms are only allowed for ATs defined in the same class.)

Type check functional dependencies as type functions.

Done:

Kind and type checking of kind signatures.

Kind and type checking of instance declarations of indexed types, including the generation of representation tycons.

Wrapper generation and type checking of pattern matching for indexed data and newtypes.